Wind turbine generators generally have a structure in which heavy objects such as blades, a gearbox, and a generator are provided at the top of a cylindrical tower having a height of several tens of meters; therefore, vibrations induced by fluctuations of wind speed are extremely large. Such vibrations increase the fatigue loading of structural components, resulting in a decrease in the life of the windmill tower.
Recently, the size of wind turbine generators has been increased. As the size of the generators increases, the effect of vibrations induced by fluctuations of wind speed becomes more significant. Thus, reducing the vibrations in wind turbine generators or windmill towers is a critical technical problem.
On the other hand, in tall structures such as high-rise buildings, active damping techniques are used in order to improve the living conditions during strong winds. Various methods have been proposed, but most of them involve driving a heavy object (mass) provided on the upper part of the structure with an actuator such as a motor to absorb the vibrations of the structure itself; one example is an active mass damper (AMD).
However, when the active damping technique (AMD) used in high-rise structures and the like is directly applied to wind turbine generators or windmill powers without modification, the following problems occur.
First, in order to achieve a satisfactory damping effect, a considerably heavy object (mass) is necessary. Furthermore, in order to drive this considerably heavy object, an actuator having a large capacity must be provided. Consequently, the weight of the nacelle significantly increases.
Secondly, since the weight of the nacelle disposed at the top of a windmill tower increases, the strength of the windmill tower supporting the nacelle must be increased accordingly. This need to significantly increase the strength of the windmill tower and other components increases the total cost of the wind turbine generator and the windmill tower.
Thirdly, an actuator for driving the heavy object (mass) is necessary. Accordingly, the number of parts for driving is increased, resulting in increased maintenance costs.
To solve the above problems, for example, Japanese Unexamined Patent Application Publication No. 2001-221145 (Patent Document 1) discloses a technique in which vibrations of a windmill tower are suppressed by providing a passive-active pitch-flap mechanism.    [Patent Document 1]    Japanese Unexamined Patent Application Publication No. 2001-221145